Bone density ligand, Sclerostin, directly interacts with LRP5 but not LRP5G171V to modulate Wnt activity

Control of the SOST bone enhancer by PTH using MEF2 transcription factors

Expression of the osteocyte-derived bone formation inhibitor sclerostin in adult bone requires a distant enhancer. We show that MEF2 transcription factors control this enhancer and mediate inhibition of sclerostin expression by PTH.

INTRODUCTION

Sclerostin encoded by the SOST gene is a key regulator of bone formation. Lack of SOST expression is the cause for the progressive bone overgrowth disorders sclerosteosis and Van Buchem disease. We have previously identified a distant enhancer within the 52-kb Van Buchem disease deletion downstream of the SOST gene that is essential for its expression in adult bone. Furthermore, we and others have reported that SOST expression is suppressed by PTH. The aim of this study was to identify transcription factors involved in SOST bone enhancer activity and mediating PTH responsiveness.

MATERIALS AND METHODS

Regulation of the SOST enhancer and promoter was studied by luciferase reporter gene assays. Transcription factor binding sites were mapped by footprint analysis and functional mutation analyses using transient transfections of osteoblast-like UMR-106 cells that exhibit endogenous SOST expression. Specific transcription factor binding was predicted by sequence analysis and shown by gel retardation assays and antibody-induced supershifts. Expression of myocyte enhancer factors 2 (MEF2) was detected by in situ hybridization, quantitative RT-PCR (qPCR), and immunohistochemistry. The role of MEF2s in SOST expression was assessed by reporter gene assays and siRNA-mediated RNA knockdown.

RESULTS

PTH completely suppressed the transcriptional activity of the SOST bone enhancer but did not affect the SOST promoter. A MEF2 response element was identified in the bone enhancer. It was essential for transcriptional activation, bound MEF2 transcription factors, and mediated PTH responsiveness. Expression of MEF2s in bone was shown by qPCR, in situ hybridization, and immunohistochemistry. MEF2s and sclerostin co-localized in osteocytes. Enhancer activity was stimulated by MEF2C overexpression and inhibited by co-expression of a dominant negative MEF2C mutant. Finally, siRNA-mediated knockdown of MEF2A, C, and D suppressed endogenous SOST expression in UMR-106 osteoblast-like cells.

CONCLUSIONS

These data strongly suggest that SOST expression in osteocytes of adult bone and its inhibition by PTH is mediated by MEF2A, C, and D transcription factors controlling the SOST bone enhancer. Hence, MEF2s are implicated in the regulation of adult bone mass.

Interaction of bone morphogenetic proteins with cells of the osteoclast lineage: review of the existing evidence

The present review evaluates the existing scientific proofs of this supplementary role of the BMPs and summarises its clinical implications. Bone regeneration is a process consisting of bone formation and bone resorption, two different but closely coupling pathways, which in most circumstances proceed simultaneously. Plenty of evidence has also characterised the bone morphogenetic proteins (BMPs) as inducing factors of bone formation. However, there is also evidence that these multifunctioning proteins affect bone resorption and the osteoclast homeostasis utilising various pathways. The present review evaluates the existing scientific evidence of this supplementary role of the BMPs, and summarises its clinical implications.

Patients with high bone mass phenotype exhibit enhanced osteoblast differentiation and inhibition of adipogenesis of human mesenchymal stem cells

Genetic mutations in the LRP5 gene affect Wnt signaling and lead to changes in bone mass in humans. Our in vivo and in vitro results show that activated mutation T253I of LRP5 enhances osteogenesis and inhibits adipogenesis. Inactivating mutation T244M of LRP5 exerts opposite effects.

INTRODUCTION

Mutations in the Wnt co-receptor, LRP5, leading to decreased or increased canonical Wnt signaling, result in osteoporosis or a high bone mass (HBM) phenotype, respectively. However, the mechanisms whereby mutated LRP5 causes changes in bone mass are not known.

MATERIALS AND METHODS

We studied bone marrow composition in iliac crest bone biopsies from patients with the HBM phenotype and controls. We also used retrovirus-mediated gene transduction to establish three different human mesenchymal stem cell (hMSC) strains stably expressing wildtype LRP5 (hMSC-LRP5(WT)), LRP5(T244) (hMSC-LRP5(T244), inactivation mutation leading to osteoporosis), or LRP5(T253) (hMSC-LRP5(T253), activation mutation leading to high bone mass). We characterized Wnt signaling activation using a dual luciferase assay, cell proliferation, lineage biomarkers using real-time PCR, and in vivo bone formation.

RESULTS

In bone biopsies, we found increased trabecular bone volume and decreased bone marrow fat volume in patients with the HBM phenotype (n = 9) compared with controls (n = 5). The hMSC-LRP5(WT) and hMSC-LRP5(T253) but not hMSC-LRP5(T244) transduced high level of Wnt signaling. Wnt3a inhibited cell proliferation in hMSC-LRP5(WT) and hMSC-LRP5(T253), and this effect was associated with downregulation of DKK1. Both hMSC-LRP5(WT) and hMSC-LRP5(T253) showed enhanced osteoblast differentiation and inhibited adipogenesis in vitro, and the opposite effect was observed in hMSC-LRP5(T244). Similarly, hMSC-LRP5(WT) and hMSC-LRP5(T253) but not hMSC-LRP5(T244) formed ectopic mineralized bone when implanted subcutaneously with hydroxyapatite/tricalcium phosphate in SCID/NOD mice.

CONCLUSIONS

LRP5 mutations and the level of Wnt signaling determine differentiation fate of hMSCs into osteoblasts or adipocytes. Activation of Wnt signaling can thus provide a novel approach to increase bone mass by preventing the age-related reciprocal decrease in osteogenesis and increase in adipogenesis.

Wise retained in the endoplasmic reticulum inhibits Wnt signaling by reducing cell surface LRP6

The Wnt signaling pathway is tightly regulated by extracellular and intracellular modulators. Wise was isolated as a secreted protein capable of interacting with the Wnt co-receptor LRP6. Studies in Xenopus embryos revealed that Wise either enhances or inhibits the Wnt pathway depending on the cellular context. Here we show that the cellular localization of Wise has distinct effects on the Wnt pathway readout. While secreted Wise either synergizes or inhibits the Wnt signals depending on the partner ligand, ER-retained Wise consistently blocks the Wnt pathway. ER-retained Wise reduces LRP6 on the cell surface, making cells less susceptible to the Wnt signal. This study provides a cellular mechanism for the action of Wise and introduces the modulation of cellular susceptibility to Wnt signals as a novel mechanism of the regulation of the Wnt pathway.

Wnt/beta-catenin signaling is a component of osteoblastic bone cell early responses to load-bearing and requires estrogen receptor alpha

The Wnt/beta-catenin pathway has been implicated in bone cell response to their mechanical environment. This response is the origin of the mechanism by which bone cells adjust bone architecture to maintain bone strength. Osteoporosis is the most widespread failure of this mechanism. The degree of osteoporotic bone loss in men and women is related to bio-available estrogen. Here we report that in osteoblastic ROS 17/2.8 cells and primary osteoblast cultures, a single short period of dynamic mechanical strain, as well as the glycogen synthase kinase-3beta (GSK-3beta) inhibitor LiCl, increased nuclear accumulation of activated beta-catenin and stimulated TCF/LEF reporter activity. This effect was blocked by the estrogen receptor (ER) modulators ICI 182,780 and tamoxifen and was absent in primary osteoblast cultures from mice lacking ERalpha. Microarray expression data for 25,000 genes from total RNA extracted from tibiae of wild-type mice within 24 h of being loaded in vivo showed differential gene regulation between loaded and contralateral non-loaded bones of 10 genes established to be involved in the Wnt pathway. Only 2 genes were involved in loaded tibiae from mice lacking ERalpha (ERalpha(-/-)). Together these data suggest that Wnt/beta-catenin signaling contributes to bone cell early responses to mechanical strain and that its effectiveness requires ERalpha. Reduced effectiveness of bone cell responses to bone loading, associated with estrogen-related decline in ERalpha, may contribute to the failure to maintain structurally appropriate bone mass in osteoporosis in both men and women.

Wnt signaling and the regulation of bone mass

Human genetic studies have firmly established a link between bone mass in humans and gain-of-function or loss-of-function mutations in a Wnt coreceptor, low-density lipoprotein receptor-related protein 5 (LRP5), or in the Wnt antagonist sclerostin, and several molecular genetic studies in mice have consistently confirmed the critical importance of the Wnt signaling pathway in skeletal biology and disease. In what may be a novel paradigm, the ubiquitous nature of LRP5/6 and Wnt signaling is counterbalanced by the bone-restricted and regulated expression of Wnt antagonists such as sclerostin and Dickkopf-1 (Dkk1) in adult tissues, offering new and potentially safe therapeutic means of intervention to stimulate bone formation.

The genetics of low-density lipoprotein receptor-related protein 5 in bone: a story of extremes

A few years ago, human genetic studies provided compelling evidence that the low-density lipoprotein receptor-related protein 5 (LRP5) is involved in the regulation of bone homeostasis because pathogenic LRP5 mutations were found in monogenic conditions with abnormal bone density. On the one hand, the osteoporosis pseudoglioma syndrome results from loss of function of LRP5, whereas on the other hand, gain-of-function mutations in LRP5 cause conditions with an increased bone density. On the molecular level, these types of mutations result in disturbed (respectively, decreased and increased) canonical Wnt signaling, an important metabolic pathway in osteoblasts during embryonic and postnatal osteogenesis. This signaling cascade is activated by binding of Wnt ligand to the Frizzled/LRP5 receptor complex. In addition to the involvement of LRP5 in conditions with extreme bone phenotypes, the genetic profile of this gene has also been shown to contribute to the determination of bone density in the general population. Quite a number of studies already demonstrated that common polymorphic variants in LRP5 are associated with bone mineral density and consequently osteoporosis, a multifactorial trait with low bone mass and porous bone structure. These genetic studies together with results obtained from in vitro and in vivo studies emphasize the importance of LRP5 and canonical Wnt signaling in the regulation of bone homeostasis. Therefore, unraveling the exact mechanisms of this signaling cascade has become an important area in bone research. This review focuses on the genetics of LRP5 and summarizes the findings on monogenic bone conditions as well as the current knowledge of its involvement in the pathogenesis of osteoporosis.

Novel LRP5 missense mutation in a patient with a high bone mass phenotype results in decreased DKK1-mediated inhibition of Wnt signaling

We found a novel heterozygous missense mutation (M282V) in the LRP5 gene in a patient with a high bone mass phenotype. In vitro studies suggest that a reduced antagonistic effect of DKK1 on canonical Wnt signaling contributes to the molecular effect of this mutation and its pathogenic consequence.

INTRODUCTION

Gain-of-function mutations in the gene encoding LDL receptor-related protein 5 (LRP5) cause high bone mass. Recent studies revealed that a reduced inhibition of canonical Wnt signaling by Dickkopf 1 (DKK1) contributes to the pathophysiology of this disease phenotype.

MATERIALS AND METHODS

We report on a 55-yr-old female patient with a high bone mass phenotype. Sequencing of exons 2-4 of the LRP5 gene was carried out to screen for disease-associated mutations in genomic DNA of the patient. The effect of the identified mutation on LRP5 membrane trafficking was studied by immunoblotting of a truncated form of LRP5. Additionally, Wnt signal activation in the absence and presence of DKK1 was assessed using a TCF4-based reporter gene assay in Saos-2 cells.

RESULTS

Our patient presents with dense bones (Z-scores > +6), and radiographic examination showed a generalized thickening of the skeleton. BMD at the hip and lumbar spine significantly decreased through the passage to menopause, indicating no protection to bone loss. Further clinical evaluation revealed torus palatinus. Mutation analysis showed the presence of a novel heterozygous missense variant (844A-->G; M282V) in LRP5, located in the first beta-propeller domain of the extracellular portion. Although protein secretion seemed to be impaired, this mutant was able to transduce Wnt signals at levels comparable with wildtype LRP5. We additionally observed a less efficient inhibition of canonical Wnt signaling by DKK1.

CONCLUSIONS

Like all high BMD-associated gain-of-function LRP5 mutations described thus far, the M282V variant affects an amino acid located in the first beta-propeller domain, underlining the functional importance of this region in the pathophysiology of these conditions. This mutation most likely alters a region important for LRP5 modulation by DKK.

SOST expression is restricted to the great arteries during embryonic and neonatal cardiovascular development

Spatial-temporal regulation of bone morphogenetic protein (BMP) and Wnt activity is essential for normal cardiovascular development, and altered activity of these growth factors causes maldevelopment of the cardiac outflow tract and great arteries. In the present study, we show that SOST, a Dan family member reported to antagonize BMP and Wnt activity, is expressed within the medial vessel wall of the great arteries containing smooth muscle cells. The ascending aorta, aortic arch, brachiocephalic artery, common carotids, and pulmonary trunk were all associated with SOST expressing smooth muscle cells, while the heart itself, including the valves, and more distal arteries, that is, pulmonary arteries, subclavian arteries, and descending aorta, were negative. SOST was expressed from embryonic day 15.5 up to the neonatal period. SOST expression, however, did not correspond with inhibition of Smad-dependent BMP activity or beta-catenin-dependent Wnt activity in the great arteries. Activity of both signaling pathways was already down-regulated before induction of SOST expression.